Model 990

Torque Magnetometer

Description

The ADE Magnetics Model 990 Torque Magnetometer is designed to automatically measure magnetic anisotropy of materials as a function of rotation angle and applied field. The advanced design has made operation and interpretation on test results efficient and uncomplicated.

Rugged Mechanical Design

Fragile torque balance mechanisms have hampered the use of torque magnetometers since their invention. Most conventional mechanisms used galvanometer movements, quartz fibers, or thin wires to suspend the sample in the magnetic field. The ADE Magnetics design has overcome the fragility problem by using a 1"-diameter air bearing assembly to suspend the sample with virtually zero friction. The air bearing assembly is rugged and capable of withstanding several pounds of lateral or vertical force without damage.

Wide Dynamic Range

The dynamic range of conventional torque balances in limited to 40 to 60 dB because of the friction and hysteresis effects shown by galvanometer movement, quartz fiber, and thin wire suspension systems. The ADE Magnetics air bearing torque balance has a practical measurement range of from .04 dyne-cm to 4000 dyne-cm, or 100 dB. Other systems require several torque mechanisms to cover such a wide range of samples, whereas the ADE Magnetics system only needs one mechanism.

Signal Processing Software

The rotational hysteresis energy loss for each cycle of torque vs. angle, rotational hystresis energy integral [ Rh ], and maximum anisotropy field are calculated. A plot of the rotational hysteresis energy loss vs. inverse field may be generated.

FFT-based signal processing software will automatically compute the anisotropy constants from the torque vs. angle data. CRT, printer, or multi-color plots of the torque vs. harmonic number and hard-copy printout of FFT coefficients are also provided.

Torque vs. angle data may be harmonically filtered to further reduce displayed noise, if desired. The maximum harmonic to be passed may be specified, and then the inverse FFT function will reconstruct the torque vs. angle data.

Infinite Field Extrapolation [Miyajima Method]

In samples with very high anisotropy, the maximum available field may be insufficient to obtain a correct value for the anisotropy constant. A peak torque test at 45° from the easy axis may be automatically executed by the ADE Magnetics system, and the correct value of the anisotropy obtained by infinite field extrapolation [ Miyajima method].

Sampled Measurements and Digital Averaging

Conventional torque magnetometers rotate the electromagnet around the sample and continuously collect data. Elaborate vibration isolation mechanisms are required when making sensitive measurements to prevent coupling of the heavy electromagnet motion with the torque mechanism. To avoid this problem, ADE Magnetics uses a different technique to collect the data. Instead of rotating the electromagnet, the ADE Magnetics Torque Magnetometer rotated the sample through use of a lightweight air bearing assembly. After the rotation servo has stabilized at the programmed angle and all mechanical vibrations induced by the movement have died away, the torque exerted by the magnetic field on the sample is measured. This method avoids potentially large errors caused by eddy currents in conducting samples and substrates when the angular motion is continuous. This method also allows the torque of many samples to be averaged per data point, thereby greatly reducing the random noise that would otherwise be present in the data. This sampled data collection method is not possible with conventional rotate-the-magnet systems because analog filtering or averaging of continuously changing data will result in a time shift of the torque data relative to the angle data. One hundred averages of the torque data on the ADE Magnetics system typically result in a ten-fold reduction in displayed random noise with out introducing rotation slew-rate errors into the torque data.